The Extremely Low ENergy Antiproton ring (ELENA)
is a small synchrotron equipped with an electron cooler,
which shall be constructed at CERN to decelerate
antiprotons to energies as low as 100 keV. Scattering of
beam particles on rest gas molecules may have a
detrimental effect at such low energies and leads to
stringent vacuum requirements. Within this contribution
scattering of the stored beam on rest gas molecules is
discussed for very low beam energies. It is important to
carefully distinguish between antiprotons scattered out of
the acceptance and lost, and those remaining inside the
aperture to avoid overestimation of emittance blow-up.
Furthermore, many antiprotons do not interact at all
during the time they are stored in ELENA and hence this
is not a multiple scattering process.